The present invention relates to an apparatus for the transverse welding, especially of synthetic resin webs for the manufacture of plastic bags, sacks or the like. More particularly the invention relates to a device for forming weld seams in webs of plastic foil or film transversely to a direction of advance of the web and having a first welding beam movable back and forth and formed with a rounded welding edge turned toward the web and a second welding beam cooperating therewith and formed on a weld roller.
In the manufacture of plastic bags, sacks or the like, a synthetic resin web, e.g. of blown film or foil, having a fold along one or both longitudinal edges is generally advanced along a path at which transverse weld seams are formed between the layers of the web to separate the web into bags and to form the seams in the bags thus produced.
The first welding beam is generally provided above the foil and can be moved up and down away from and toward the web and, of course, the welding roller which forms the second beam and underlies the web. The welding roller usually has a jacket of rubber or the like.
The welding roller extends the full width of the web and it is also customary for the first welding beam provided with the aforementioned edge to extend the full length of the welding roller. In practice, however, the transverse beam which must be formed by the edge of the first beam is frequently less than the full length of the first beam and of the welding roller. It has been found that when the web is narrower than the length of the welding edge and the welding roller, significant wear occurs at the portion of the welding edge beyond the web and on the welding roller beyond the web as well. When the welding roller is coated as polytetrafluoroethylene, moreover, in the region in which the welding edge may contact this coating without an intervening synthetic resin web, wear of the coating is relatively rapid.
Uncontrollable wear and reshaping of the edge of the first welding beam and damage to the coating or jacket of the welding roller interact to promote further rapid deterioration of the welding tools. For example, uncontrolled wear of the welding edge can promote or induce additional wear of the welding roller. When, therefore, a wider web is to be used and the seam is to be formed utilizing parts of the welding edge of the welding roller which have previously been excessively worn because of the lack of a web between these parts, the weld seam can be unsatisfactory and can even cause rejection of the bags which are produced.
It is, therefore, generally the practice to change one or more of the weld beams upon changing of the width of the web to be used. As a consequence, the weld beams are frequently replaced at high cost. The replacement of the roller, for example, means that it frequently must be recoated as with the comparatively extensive material, polytetrafluoroethylene, even if it has a rubber jacket.
It is, therefore, the principal object of the present invention to provide an improved apparatus or device for the purposes described which, in a simple manner, can prevent premature wear of the welding tools and nevertheless can permit rapid matching of the welding tools to the width of the synthetic resin web which is to be seamed.
Another object is to obviate the drawbacks of earlier systems as described.
These objects are attained, in accordance with the invention, in a device for the transverse welding of synthetic resin webs for producing bags, sacks and the like, comprising a first welding beam formed with a rounded welding edge turned toward the web and a second welding beam juxtaposed with the edge and formed as a welding roller having a cylindrical carrier body extending over at least a maximum length (Lmax) of a weld seam to be formed transversely of the web, and a plurality of elements mountable on the carrier body in an axially abutting, contiguous and gap-free assembly forming a substantially continuous surface over a length (L) of a weld seam less than the maximum length.
Thus, according to the invention, the second welding beam, at least over a length corresponding to the maximum length of a weld seam to be formed in the synthetic resin web, comprises a cylindrical carrier body with a plurality of elements mounted thereon in a gapless, end-to-end manner forming the surface juxtaposed with the welding edge over the length of the welding seam which is to be formed. These elements are concentrically arranged and are directly or indirectly connectable individually or collectively to the carrier body and to the welding roller. The elements can be rings fitted over the roller tube which then forms the carrier body and the rings can be either of equal width or can comprise a base ring having a length equal to the minimum length of weld seams to be formed and adjoined by one or more additional rings which themselves can be of equal or unequal width.
The system of the invention provides a simple way of adjusting welding width to the particular web beam used without the danger that either the welding edge or the coating on the welding roller will be adversely affected by the lack of a foil web between them.
To prevent tilting of the upper welding beam, in a region corresponding to the maximum welding width, a support ring can be mounted on the carrier body and can be continuously affixed through this support. The ring can be provided at an end of the carrier body opposite the end at which the base ring is provided.
The rings which are mounted on the carrier body can have rubber jackets and can be provided on their outer peripheries with a low friction coating of a synthetic resin, especially polytetrafluoroethylene.